The present invention relates generally to the metal tube art, and, more particularly, to the manufacture of seamless, shape memory, metal tubes, especially those using nickel-titanium or titanium alloys.
Most seamless metal tubes are made by working a tube blank over a nondeformable mandrel and/or in combination with a sinking process where the tube is drawn through a die without internal support. Such discontinuous processes are slow and expensive, and can only produce tubes of limited length. It is also known to make seamless tubes of uniform cross section by mechanical working of an assembly of a core and a tube blank, thus elongating both the core and the tube blank, and then removing the core. Core removal has been achieved, depending on the core material, by melting a core which melts at a temperature below the melting point of the tube, by selectively dissolving the core, or according to a previous invention by mechanically stretching the core to a reduced diameter to facilitate core removal. Dimensional precision and internal surface quality for the deformable mandrel process are also more difficult to control as the plastic flows for the blank and the core can be different when the core is made of a different material from the tube blank. Assembly gap or clearance between the core and the tube blank can also contribute to the degradation of internal surface quality. Even when the core and the tube blank are made of the same material, it is believed that drawing friction may lead to different elongation between the tube blank and the core.
United Kingdom Patent No 362539 discloses production of hollow metal bodies.
French Patent No. 980957 discloses assembling a tube blank with a core, mechanical working reduction without bonding, further core elongation to enable longitudinal removal and then removal of the core.
U.S. Pat. No. 2,809,750 discloses a mandrel for extrusion press.
U.S. Pat. No. 4,186,586 discloses a billet and process for producing a tubular body by forced plastic deformation. In this patent the entire billet 10 is subjected to plastic deformation which includes both the center core 13 and the sheath pipe 12. There is hydrostatic co-extrusion of a metallic tube blank and metallic core separated by a solution removable salt layer. After reduction, the salt layer defines an annular gap so that after dissolving the salt, the metallic core can be longitudinally withdrawn.
U.S. Pat. No. 4,300,378 discloses a method and apparatus for forming elongated articles having reduced diameter cross-section. The billet is a solid sample and does not have a tube in connection with a mandrel. This patent shows a standard process of tube extrusion about a conical mandrel 106.
U.S. Pat. No. 4,653,305 discloses a method and an apparatus for forming metallic article by cold extrusion from a metallic blank.
JP 62199218 A (Furukawa Electric Co LTD) 2 Sep. 1987, discloses the making of shape memory alloy pipe in which a mandrel is inserted into a cylinder made of shape memory alloy, the cylinder and mandrel are reduced integrally and the mandrel is pulled out after a heat treatment. It shows co-reduction of a tubular nickel-titanium shape memory alloy blank and stainless steel core using shape memory effect of the tube material (a rolled up, welded and thickness reduced sheet) to expand the tube to enable core removal.
U.S. Pat. No. 5,056,209 discloses a process for manufacturing clad metal tubing. It shows a method of co-extruding concentric metal tubes to form a clad bimetallic tubular end product. The materials are carbon steel tubing as an outer tube and harder to work materials having higher deformation resistance.
U.S. Pat. No. 5,709,021 discloses a process for the making of metal tubes in which a seamless metal tube is made by elongating an assembly of a tube blank and a metal core by mechanical working, and then stretching the core.
Objects of the present invention are to overcome the difficulties of the prior art and to produce a better product than the prior art. These objects and others, are accomplished in accordance with the present invention which provides that these problems can be overcome by employing: (1) shape memory effect to reduce the assembly gap or clearance between the core and the blank (in the smaller formats); and (2) a drawing process which reduces or eliminates relative elongation between the core and the tube during drawing; or (3) a hybrid process comprising a deformable mandrel process for the up-stream reductions and a nondeformable mandrel process for the final finishing passes. Lubricants between the core and the tube may be beneficially used during the process. Also, there is a benefit in using decoring and reinserting, which provides the ability to fine tune the ratio at closer to the final size in order to better control final dimensions, and allows for a new lube layer to be added between the tube and core thus easing decorability for small, long tubes.
The invention can be used to make shape memory alloy such as NiTi family alloy tubes having a wide range of sizes, but is particularly useful for making thin wall tubes of small diameter, for example of inner diameter from 0.005 to 1.0 inch (0.13 to 25.4 mm), e.g., 0.005 to 0.125 inch (0.13 to 3.2 mm) and wall thickness 0.001 to 0.2 inch (0.025 to 5 mm), e.g., 0.002 to 0.1 inch (0.05 to 2.5 mm). The length of the tube can vary widely. Thus the invention can be used to make tubes of considerable length, e.g., more than 20 feet, or even more than 100 feet, with the upper limit being set by the equipment available to stretch the core.
In the smaller formats there can be improvements if a decoring and reinserting step is used.
Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings in which: